Magnetic device for use in acoustic apparatus

ABSTRACT

This invention relates to a magnetic device comprising a magnet, a center pole secured to an upper surface of said magnet, a yoke enclosing said magnet and said center pole such that a gap is formed between said yoke and said center pole, a voice coil wound on a bobbin loosely received in said gap, and a metallic member which permits the lines of magnetic force to move substantially in one direction only. Said magnetic member is mounted either on the center pole or the yoke in the vicinity of the surface of the north pole or south pole.

United States Patent 1191 Sato etal. Jan. 1, 1974 [5 MAGNETIC DEVICE FOR USE IN 2,380,616 7/1945 Snoek et al. 335/231 ACOUSTIC APPARATUS 3,549,830 12/1970 Hofman. 179/117 3,129,298 4/1964 Hofman.... 179/120 Inventors: Tadao Sato, Asaka; Toshio Ebihara, 3,550,051 12/1970 Parker 179/117 x Ichikawa, both of Japan 2,930,914 3/1960 Johnson.... 310/27 3,593,239 7 1971 H f 179 217 X [73] Assignees: Coral Audio (Forpolzatio 2,774,000 1211956 3101217 x Kawaguchi C1ty; Shmko Magnet lgatlilusltillkl Kaisha, Narashmo City, Primary Examiner-D F gg 2933,31??? An0rneyStevens, Davis, Miller & Mosher [22] Filed: Dec. 13, 1971 [21] Appl. No.: 207,081 [57] ABSTRACT This invention relates to a magnetic device comprising [30] Foreign Application Priority Data a magnet, a center pole secured to an upper surface of Dec. 19, 1970 Japan 45/114666 Said magnet, a Y enclosing Said magnet and Said center pole such that a gap is formed between said 521 U.S. c1 310/27, 179/115.5 R, 179 117 yoke and Said Center P919, 8 voice coil wound on a 51 1111.01. H02k 33/18 1 bobbin looselv'received in said p and a metallic [58] Field of Search 310/13, 27, 217; member which Permits the of magnetic force to 7 155 H9 120, 7 55 335/231 move substantially in one direction only. Said mag netic member is mounted either on the center pole or 5 References Cited the yoke in the vicinity of the surface of the north pole UNITED STATES PATENTS 3,134,057 5/1964 Tsungo et al. 179/117 X 1 Claim, 8 Drawing Figures PAIENIEDJM nan 3.733.311 SHEET 10F 2 I mmw-mm PAIENTED AN 1 i974 3,783.31 1

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FIG. 7 FIG. 8

MAGNETIC DEVICE FOR USE IN ACOUSTIC APPARATUS This invention relates to magnetic devices, such as a speaker and microphone, etc., in an acoustic apparatus, and more particularly it is concerned with a magnetic device which can reduce distortions of the vibration system due to the unbalance of magnetic field intensity in an effective air gap.

Generally, a distortion in the ranges of medium and high loudness levels of sound is believed to be caused by non-lineality of the vibration system in the split vibration zone. However, the major portion of the distortion of higher harmonics occurring in the ranges of medium and high loudness levels of sound is caused by non-lineality of the magnetic device.

In a conventional magnetic device or a speaker, for example, the magnetic field extending from a center pole 1 to a yoke 2 comprises a magnetic flux 3 in an effective gap flux and a stray flux 4 in the interior and exterior of the yoke 2, and all the flux does not pass through the effective gap. Such magnetic device has a very low efficiency.

A principal object of this invention is to reduce distortions in high harmonics, low harmonics and ampli tude, etc. caused by non-lineality of the magnetic device.

Another object of this invention is to improve the efficiency of magnetic flux in the air gap by preventing the leakage of the magnetic flux 4 which strays in the portions other than the effective air gap.

A further object of this invention is to achieve a maximum efficiency of a magnetic device at low cost.

In order to accomplishthe aforementioned objects, this invention provides a magnetic device which comprises at least one member made of a metal which permits the lines of magnetic force to move, substantially in one direction and which has low iron loss, such metallic member being provided in the vicinity of the surfaces of the north pole or the south pole of the magnetic device juxtaposed to a gap so that a unidirectional magnetic field is produced and the leakage of an internal or external stray magnetic flux from the effectivegap is markedly reduced by the action of said metallic member, whereby the efficiency of the magnetic flux in the effective gap can be improved and a distortion of higher harmonics in the audio band can be eliminated.

Additional objects as well as features and advantages of this invention will be evident from the description set forth hereinafter when considered in conjunction with the accompanying drawings, in which:

FIG. 1 is a view in explanation of a conventional mag netic device;

FIG. 2 is a sectional view of one embodiment of the magnetic device according to this invention;

FIG. 3 is a sectional view, on an enlarged scale, of essential portions of the magnetic device of FIG. 2; and

FIG. 4 to FIG. 8 are sectional views of the other embodiments of this invention.

In FIG. 2, there is shown a magnet 5. A center pole 1 is secured to the upper portion of the magnet 5. The magnet 5 and center pole 1 are secured to a yoke 2 such that they are enclosed by the [Ll-shaped yoke 2. The upper end portion of the center pole l is received in a circular opening 6 formed in the yoke with a gap 7 being formed therebetween.

A uni-directional metallic member 8 is disposed in the vicinity of the surfaces of the yoke 2 and center pole l opposed to each other. As the uni-directional metallic member 8, a silicon-steel plate may be used, for example, which is adapted to cause the lines of magnetic force to move substantially in one direction only by having its crystals oriented in the direction and which has high specific resistance and low iron loss.

The center pole 1 is supported by a centering 9 secured to the underside of a top plate of the yoke 2. A bobbin 11 having a voice coil 10' wound thereon is loosely mounted in the gap 7 for vertical movement.

A conical frame 12 is secured to an upper surface of the yoke 2, and a cone 14 is fitted between an upper marginal portion of the frame 12 and a packing 13. The cone 14 is bonded at its center lower end portion to the bobbin 11 at its outer side surface. A damper 15 is secured at its inner peripheral edge to the outer side surface of the bobbin 11, and at its outer marginal edge to the frame 12. A dust prevention cover 16 is mounted on an upper portion of the bobbin l1.

Leads 17 of the voice coil 10 extend through the cone l4 and frame 12 to outside.

The uni-directional metallic member 8 may have, for example, a property such that it permits the lines of magnetic force to move substantially crosswise thereof. It may be mounted in doughnut shape on upper and lower surfaces of the yoke 2 in disk shape on an upper surfaces of the enter pole l, and cylindrically in a lower portion of the yoke 2 except for a surface thereof which is juxtaposed to the circular opening 6. Alternatively, the members 8 may be mounted in a plurality of these positions.

It will be appreciated that, in the present invention, the uni-directional metallic member 8 which has low iron loss and permits the lines of magnetic force to move substantially in one direction only is provided in the vicinity of the surfaces of the north pole and the south pole of a magnetic device which are juxtaposed to each other so that the eddy current loss is reduced, the specific magnetic permeability is increased and the inner and outer stray magnetic flux is eliminated. The magnetic field in the magnetic device is substantially confined to the magnetic field 3 that passes through the effective gap 7, with a result that a distortion of higher harmonics in the audio band is reduced and efficiency is markedly increased.

FIG. 4 shows an embodiment in which the center pole 1 is made of a material of high magnetic permeability. The use of a material of high magnetic permeability is conducive to increased magnetic flux density but not conducive to the elimination of a distortion of higher harmonics. The provision of the uni-directional metallic member 8 on an upper surface of the center pole 1 thus permits to markedly eliminate a stray magnetic flux.

FIG. 5 shows an embodiment in which the unidirectional metallic member 8 is only provided on an upper marginal portion of the yoke 2.

In the embodiment shown in FIG. 6, the unidirectional metallic members 8 are provided on the surfaces of the yoke 2 and center pole 1 respectively which are opposed to each other with the effective gap 7 being interposed therebetween, so that the movement of the lines of magnetic force horizontally between the yoke and center pole is facilitated. Alternatively, the unidirectional metallic member 8 may be only provided on the surface of the yoke 2 or the surface of the center pole 1.

FIG. 7 shows an embodiment in which the magnet itself serves as a center pole, and the uni-directional metallic members 8 on an upper surface of the yoke 2 and on an upper surface of the magnet 5 respectively. The manner of mounting the members 8 in this embodiment is not limited to the one illustrated, and the member or members 8 may be mounted in any position or positions as desired as shown in the embodiments of 10 FIGS. 3 to 6.

In FIG. 8, there is shown an embodiment in which the center pole 1 is made of the same material as the unidirectional metallic member. The center pole 1 is formed such that it is in an inverted cone shape and bent inwardly so that the center pole 1 may face a direction which agrees with the direction of movement of the lines of magnetic force.

From the foregoing description, it will be appreciated that the magnetic device according to this invention comprises at least one uni-directional metallic member disposed in the vincity of the surfaces of the north pole and south pole of the magnetic device which are juxtaposed to the gap either on the north pole side or the south pole side so that the leakage of an internal or external stray magnetic flux from the effective gap is prevented. This arrangement is effective to make the best use of the energy of the magnet and increase the efficiency of the magnetic device, whereby non-linear distortions and various other distortions occurring in a magnetic device used as an electro-acoustic or acoustico-electric transducer with a speaker or microphone can be eliminated. The magnetic device provided by this invention has an additional advantage in that it is simple in construction and low in cost.

What is claimed is:

1. A magnetic device, comprising: means forming a magnetic circuit and defining a magnetic gap between two opposed surfaces thereof, said magnetic circuit forming means including a magnet,

a center pole secured to an upper surface of said magnet, and

a yoke enclosing said magnet and said center pole such that said magnetic gap is formed between said yoke and said center pole; acoustic vibration producing means disposed in said gap; at least one uni-directional metallic member composed of silicon steel, having low iron loss and permitting movement of magnetic lines of force generated by said magnetic circuit substantially in only one direction, mounted on at least one of said two opposed surfaces adjacent said magnetic gap, wherein the magnetic field generated by said magnetic circuits is confined substantially to that field which passes directly between said opposed surfaces defining said magnetic gap and wherein said at least one metallic member is disposed on the top surface of said center pole adjacent said gap; further metallic members disposed on opposed surfaces of said yoke adjacent said gap; and a still further metallic member disposed on a side surface of said center pole adjacent to and outside said gap. 

1. A magnetic device, comprising: means forming a magnetic circuit and defining a magnetic gap between two opposed surfaces thereof, said magnetic circuit forming means including a magnet, a center pole secured to an upper surface of said magnet, and a yoke enclosing said magnet and said center pole such that said magnetic gap is formed between said yoke and said center pole; acoustic vibration producing means disposed in said gap; at least one uni-directional metallic member composed of silicon steel, having low iron loss and permitting movement of magnetic lines of force generated by said magnetic circuit substantially in only one direction, mounted on at least one of said two opposed surfaces adjacent said magnetic gap, wherein the magnetic field generated by said magnetic circuits is confined substantially to that field which passes directly between said opposed surfaces defining said magnetic gap and wherein said at least one metallic member is disposed on the top surface of said center pole adjacent said gap; further metallic members disposed on opposed surfaces of said yoke adjacent said gap; and a still further metallic member disposed on a side surface of said center pole adjacent to and outside said gap. 